While it is pretty easy to build an antenna for 40 MHz, the difficulties of mechanical strength and waterproofing are always a problem. Some may considering buying a commercial antenna and this is an example of one from SIRIO.
The SIRIO GPA is a basic quarter wave ground plane antenna and there are several models available to cover various frequency ranges.
GPA 27-45: 27 … 45 MHz Tunable
GPA 40-70: 40 … 70 MHz Tunable
GPA 66-108: 66 … 108 MHz Tunable
GPA 108-136: 108 … 136 MHz Tunable
GPA 135-175: 135 … 175 MHz Tunable
GPA 170-230: 170 … 230 MHz Tunable
As there are no loading coils, it's just a case of adjusting the length of the vertical radiating element and each of the three ground plane radials.
The chart below shows the length for 'L'. This is roughly 1.8 metres for 40 MHz.
This is the frequency response for the 40-70 model but it should be pretty much the same as the 27-45 model.
The 1:2.0 VSWR bandwidth seems to be around 4.0 MHz
Note that the base of the antenna is mounted on the top of a pole as opposed to the side and is secured by screws. As a result, the coax will be inside the support mast.
If this doesn't suit, an optional adapter may be purchased.
While a quarter wave ground plane isn't a top performer, it does have the advantage of being omni-directional.
If this antenna is mounted at a height and in the clear, it should be an excellent antenna for monitoring the low VHF spectrum between 30 and 50 MHz.
1) GPA models on SIRIO website